For the separation of suspended solids or dissolved macromolecules from a fluid by filtration, two basic configurations are used, direct-flow filtration (also called normal flow filtration) and crossflow filtration (also called tangential flow filtration).
In direct-flow filtration, fluid is introduced perpendicularly to the filter surface and then passes directly through the filter.
In crossflow filtration, on the other hand, a fluid flow is passed tangentially along the filter surface. Particles/molecules smaller than the pore size of the filter pass through the membrane as a permeate (filtrate), while everything else is retained on the feed side of the membrane as a retentate. Usually the fluid flow is recirculated across the filter surface. Since the retained products are swept along the surface by the tangential flow and do not build up at the filter surface as in direct-flow filtration, a crossflow filter can operate continuously at relatively high solids loads without being blocked by solids material.
A crossflow filtration system typically includes a filtration module, a feed tank, a pump for feeding liquid from the feed tank to the filtration module via a feed line, a return line for circulating retentate back to the feed tank, a valve in the retentate return line for applying pressure, and a permeate line for removing permeate from the filtration module.
Depending on the pore size of the filter, typically a membrane, crossflow filtration is classified as either a microfiltration (MF) or ultrafiltration (UF) process. Filter membranes are commonly configured as flat-sheet filter cassettes, which typically comprise a stacked assembly of porous membrane sheet components and permeate and retentate flow screen components, as described in e.g. US Pat. Appl. No. 2008/0264852 and U.S. Pat. No. 4,735,718. In a filtration module, several such filter cassettes are usually clamped together in a crossflow filtration cassette holder, as described in e.g. U.S. Pat. No. 7,635,426.
A crossflow filtration cassette holder typically includes a distributor plate, which has retentate and permeate inlet and outlet apertures in contact with the corresponding apertures on a cassette clamped towards the distributor plate. The retentate inlet apertures on the distributor plate are usually connected with a retentate inlet channel inside the plate, which ends in a retentate inlet connector. Similarly, the retentate outlet, the permeate inlet and the permeate outlet apertures are connected with the corresponding channels inside the plate, each ending in a connector. The distributor plate is joined to an end plate with tie rods, forming a cage-like structure within which a number of cassettes are placed with their apertures in registry and the tie rods engaging in indentations on the cassette sides to lock them in place. The cassette stack is then clamped to the distributor plate to obtain efficient sealing, e.g. by a compressor plate between the end plate and the cassette stack.
The assembly and disassembly of the holder can be time-consuming, involving the dismounting of the end plate from all the tie rods. An alternative is to include a hinged tie bar in the structure, which can be opened to allow mounting and removal of cassettes without disassembly of the holder. Such a solution is described in WO 2010/151212 FIG. 2. The hinged structure is however mechanically sensitive and expensive to manufacture and there is thus a need for an improved solution.